1. Field of the Invention
The present invention relates to disk storage drives such as magnetic disk units and optical disk units in which high-precision head positioning control is realized by letting a fine-movement actuator (of high response and short stroke) and a coarse-movement actuator (of low response and long stroke) operate in cooperation, and in particular, to positioning control technology for such a two-stage actuator by which input-output characteristics of the fine-movement actuator can be estimated and identified.
2. Description of the Related Art
A disk storage drive such as a magnetic disk unit is required to drive its head to a desired track of a disk by an actuator at high speed (seek control) and let the head stay on the desired track with high accuracy (following control) in order to realize high-speed access to information that has been recorded on the disk with high data density. To meet such requirements, it becomes necessary to reduce the weight of the actuator and widen the frequency band of the positioning control system. On the other hand, there is a certain limit to the increasing of resonance frequency of the actuator mechanism since weight reduction deteriorates the stiffness of the actuator, by which the frequency band of the positioning control system is necessitated to be restricted.
The track width of disks supported by existing magnetic disk units is approximately 0.5 μm, and positioning accuracy of the magnetic disk units is approximately 0.05 μm. Magnetic disk units of the future are required to deal with a track width of 0.2 μm or less, for which positioning accuracy of 0.02 μm or less is required to be attained. For such positioning accuracy, the frequency band of the positioning control system has to be widened from approximately 1 kHz of the present to 3 kHz or more. As a technique for widening the control frequency band, “two-stage actuator method” has become known, in which a coarse-movement actuator implemented by a voice coil motor is combined with a fine-movement actuator implemented by a piezoelectric element etc. and thereby the magnetic head is positioned with high accuracy. In general, the maximum driving voltage applied to the fine-movement actuator (piezoelectric element etc.) is approximately ±30 V, by which the head position changes approximately ±1 μm. However, the relationship between the driving voltage and the moving distance of the head (input-output characteristics of the actuator) varies due to variations in the manufacturing process of the piezoelectric elements, variation of the piezoelectric element with time, environmental conditions such as temperature and humidity, etc.
Meanwhile, a positioning control technique estimating the input-output characteristics of the fine-movement actuator has been disclosed in JP-A-2000-285621, for example.
The two-stage actuator is operated by two control inputs: a first input to the coarse-movement actuator and a second input to the fine-movement actuator. Meanwhile, a head position signal indicating the total moving distance of the head, obtained by adding the moving distance of the coarse-movement actuator to that of the fine-movement actuator, is outputted as an output signal. As a matter of fact, the deviation of the head moving distance from position information which has previously been recorded on the disk is detected and the deviation is outputted as the output signal, as will be described in detail. Therefore, the two-stage actuator as a controlled system is a two-input one-output system, and some contrivance becomes necessary for realizing the estimation of the input-output characteristics of the fine-movement actuator. As for the coarse-movement actuator, its input-output characteristics (gain) can be estimated easily using conventional identification methods, by interrupting the driving voltage to the fine-movement actuator and letting the coarse-movement actuator operate alone (letting the two-stage actuator operate as a one-input one-output system). On the other hand, for the estimation of the input-output characteristics of the fine-movement actuator, it is difficult to let the fine-movement actuator operate alone for the positioning without letting the coarse-movement actuator operate, since the stroke of the fine-movement actuator is too small relative to various external perturbations to the actuator. Therefore, only the total moving distance of the coarse-movement actuator and the fine-movement actuator can be observed and obtained from the head position signal as the output of the actuator.
In the aforementioned JP-A-2000-285621, a method for estimating the gain of the fine-movement actuator has been proposed. In the method, the gain of an error rejection rate (sensitivity function) of the coarse-movement actuator at a specific frequency is measured first. Subsequently, the gain of the fine-movement actuator is estimated by actually measuring a gain of the fine-movement actuator (regarding its driving input signal and the head position signal) at the aforementioned specific frequency and multiplying the measured gain by the gain of the error compression rate of the coarse-movement actuator. The method, requiring two measurement steps for the estimation and employing a Fourier-transform technique for the gain estimation, involved problems with estimation time and estimation accuracy. Further, an external perturbation signal like a sine wave has to be applied to the fine-movement actuator for the estimation, during which data reading/writing by the head becomes impossible. To sum up, various problems remain to be solved for the estimation of the input-output characteristics of the fine-movement actuator.